Ceramic firing techniques are known in which the material to be fired, especially clay which is capable of expanding, is placed in firing molds preferably in the form of granulates or pellets and is heated therein until melting and softening of the material as well as pyroplastic adhesion takes place, (German Pat. No. 1 914 372) at least at the surface of the material to be further fired at high temperature.
In order to be able to manufacture these bodies, firing molds have become known which are sufficiently heat-resistant and capable of withstanding changes in temperature and in which there are no significant contact reactions or chemical bonding with the material to be fired (German Offenlegungsschrift No. 21 55 933). However, despite careful selection of the material and shaping of the parts of the firing mold, bonding phenomena due to rough machining or moistening of the components of the mold by molten portions in the material cannot be entirely eliminated. These phenomena which occur, particularly in the case of localized overheating, lead to impurities or completely ruin the firing molds with respect to their further use and moreover lead to considerable difficulties in removing the molded body from the mold after the firing process has been terminated, i.e. in separating the molded body from the parts of the mold without damage or without parts of the material to be further fired sticking to the mold.
The above-mentioned bonding phenomena stops when the material is cold or during the cooling-off process of the material and in fact stops solely due to the differing thermal contraction of the material and the firing mold when a temperature has been reached which is below the quartz transition temperature of the material. However, for economic reasons, it is advisable to remove the material at as high a temperature as possible since if the firing mold is allowed to cool reheating of the mold is necessary during the following firing process and this leads to considerable loss of energy and restrictions in the material used as the mold material.
If removal from the mold is carried out bearing in mind the cost efficiency at high temperature, there is the difficulty of removing the molded body from the firing mold without irreparable damage, thereto if in some circumstances due to local overheating at areas of contact with the material and the mold, the mold has been moistened by the viscous material and bonding to the mold takes place. A further difficulty of removal of the body from the mold arises from the fact that the softened molded body catches mechanically on the component parts of molds having more complicated mold components.
Practical tests have shown that the bonding forces which arise due to moistening or catching may be so large that the body cannot be removed from the firing mold by lifting or sliding without damaging the areas of contact.